Gratings are very important building blocks in various optical systems. They are implemented in a wide field of applied optics and industrial applications. A great number of modern optical devices and systems implement all kinds of optical gratings since many photo spectroscopic devices, lasers, solar modules and systems, optical waveguides, wave filters, optical sensors, rely on the efficient redirection of light as a function of the wavelength of that light. Diffraction gratings may be used as a device to change the angle of an incident beam, to separate light in its different spectral components, to mix beams with different wavelengths such as in a telecom multiplexer or to filter a part of the spectrum of a light beam such as in a monochromator or spectrometer.
Diffraction gratings find also more and more applications in security devices and in display applications. They can also be found in fiber optical sensors, for example in the form of distributed Bragg gratings. For polarisation applications as well as applications using the interaction of plasmons with photons, metallic gratings have also been widely considered.
Numerous basic publications in the literature focus on the physics, the fabrication technologies and the applications of gratings such as:    M. Born and E. Wolf, “Principles of Optics”, Pergamon Press, Oxford, 1993.    R. Petit, “Electromagnetic Theory of Gratings”, R. Petit, ed. Springer Verlag, berlin 1980.    M. Schnieper et al., “Application and fabrication of subwavelength gratings” in Diffractive Optics and Micro-optics, Technical Digest (Optical Society of A. Erica, Tucson 2002), p. 228-230.
Many publications exist that describe how to improve the efficiency of gratings and grating structures, how to miniaturise and fabricate them and since two decades a lot of effort has also been put in the development of replication methods of grating structures.
The standard approach to redirect light by the diffraction effect is by the use of periodic microstructures such as linear shaped microgrooves fabricated on the surface of a glass or metallic surface. More than a century ago the first gratings were simple groove shaped lines and were realised by diamond scribing techniques requiring huge and complicated mechanical machines. Today these microgrooves can be fabricated by a wide variety of techniques, such as diamond machining, etching, nanoimprint or deposition techniques, and by modern technology complicated shapes of the grooves may be realised, such as multilevel diffractive optical elements or DOE's. It has been a constant trend to improve gratings by developing new shapes, new arrangements of gratings and by using special materials having specific properties such as the index of refraction or the combination of different materials so as to obtain special optical effects with the gratings.
In order to improve the efficiency of the diffraction of light by a grating, asymmetric grating profiles of substantially linear shaped grooves have been considered in the past, with cross sections (i.e. perpendicular to the groove line) of the grating elements having a shape such as sawtooth-, slanted-, binary blazed- or multilevel step shape. The shape of the grating structures that diffract light are in general optimized for a specific incidence angle of the light impinging on the grating, most common a normal incidence angle, and one preferred diffraction order, typically the first or second diffraction order, although higher orders can be considered also such as in an echelle grating wherein the diffraction order can be very high, for example 30 or 80. For low order diffraction effects the efficiency can be achieved by making a blazed-type diffraction grating, which is designed, to have a substantial trapezoidal shaped grating element cross sections, inclined surfaces of which, with respect to the normal incidence angle to the grating, are arranged so as to diffract light in a specific predetermined direction. All of the above mentioned gratings however are either challenging or costly to fabricate or to replicate.
The diffraction efficiency is a parameter that is constantly improved as a particular diffraction order is used, and the rest of the transmitted, scattered or diffracted light is mostly perturbing light for the system. So, one seeks constantly to improve the diffraction efficiency. A fundamental reason for the limitation of the diffraction efficiency for a specific order, for example the first diffraction order, is related to the fact that when light incident perpendicularly on a symmetric diffraction grating is diffracted, at most 50% of the diffracted light can be directed to for instance the first positive and the first negative diffraction order each, for symmetry reasons. Although blazed type transmission gratings enhance considerably the diffraction efficiency for a specific wavelength, a difficulty arises when dealing with multi-color light, such as in displays, because the diffraction efficiency of the other wavelengths is reduced. In transmission diffraction gratings a fraction of the light is transmitted in the zero-order, which in most applications is a source of undesirable light, reducing contrast or leading to perturbations of signals in sensors. Accordingly, to enhance the diffraction efficiency, it is desirable to reduce zero-order transmitted light.
A structure that reduces the zero order transmitted light of gratings is disclosed in US 2005/0078374 wherein a partial metallisation of the grating element reduces the zero order light. The disclosed structure uses blazed type grating structures, which are difficult to fabricate and reproduce. The structure disclosed in US 2005/0078374 still has a basic limitation in the diffraction orders other than the zero order.
More specifically, gratings may be used to couple light efficiently into a waveguide or a window. Different grating structures have been proposed such as blazed gratings to improve the light coupling in optical waveguides, such as in WO 2010/122329 wherein blazed grating elements are provided on the surface of the waveguide. As the grating elements are blazed, their realisation is difficult and the diffraction efficiency stays limited by the above-mentioned fundamental limitations.
In other approaches to enhance the light coupling by grating couplers in a waveguide dielectric coatings have been used on the grating elements.
Other methods have been proposed to improve the lightcoupling efficiency of binary, non-blazed, gratings such as described by S. Siitonen et al in their article “A double-sided grating coupler for thin waveguides”, Opt. Express 2007, Mar. 5, 15(5), 2008-18.
Although crossed binary gratings, arranged on opposite sides of a thin waveguide, improve the light coupling efficiency in the waveguide, the solutions is still of limited use as the crossed gratings have to be arranged on the two sides of a thin waveguide, and the solution would not be possible for a thick waveguide and certainly not for a window. Also, it requires a more complicated, and thus more expensive, solution than using a binary grating arranged on one side of a waveguide.
Another method to couple light efficiently in a waveguide is to arrange a Bragg grating arranged along a waveguide so as to enhance the coupling efficiency in the waveguide. Such a method is described in U.S. Pat. No. 4,737,007. Although the proposed structure improves the incoupling efficiency in a waveguide, its application is limited to thin waveguides as it is based on interference effects and guided mode resonances. Their approach would not be suited to enhance considerably the incoupling efficiency in a waveguide or optical window in which there is no distributed interaction between the grating structure and the diffracted light by that type of grating structure. Also, the incoupling efficiency enhancement in the structure proposed by U.S. Pat. No. 4,737,007 is limited to a narrow wavelength band, i.e. some angstroms.